1. Field of the Invention
The present invention relates to an image pickup apparatus such as a digital camera or a video camera, and more particularly to an image pickup apparatus which performs focus detection by using signals from pixels in an image pickup element.
2. Description of the Related Art
Japanese Patent Laid-Open Nos. 2000-156823 and 2000-292686 disclose image pickup apparatuses each of which includes partial pixels having different optical characteristics from those of other pixels in entire pixels constituting an image pickup element used in the image pickup apparatus, and performs focus detection based on outputs from the partial pixels. In more detail, each of these image pickup apparatuses includes plural pairs of focus detection pixels among plural image pickup pixels on the image pickup element, and performs focus detection of a phase difference detection method (hereinafter referred to as “phase difference detection method focus detection”) by using signals from the pairs of the focus detection pixels. Then, the apparatus calculates an in-focus position (or a driving amount) of a focus lens based on a focus detection result, and moves the focus lens according to a calculation result to perform focusing.
Japanese Patent Laid-Open No. 2001-305415 discloses an image pickup apparatus which vertically divides a light receiving portion of each of partial pixels in an image pickup element into two, and performs phase difference detection method focus detection for an object having a vertical luminance distribution by using signals from the two-divided light receiving portions. This image pickup apparatus further detects contrast of an object having a horizontal luminance distribution by pixels horizontally adjacent to each other to perform focus detection of a contrast detection method (hereinafter referred to as “contrast detection method focus detection”).
Japanese Patent Laid-Open No. 2003-153291 discloses an image pickup apparatus in which focus detection pixels having light receiving portions each being divided horizontally or vertically are repeatedly disposed every other line in an image pickup element, and which performs phase difference detection method focus detection for an object having a horizontal luminance distribution or a vertical luminance distribution.
However, in the image pickup apparatuses disclosed in Japanese Patent Laid-Open Nos. 2000-156823, 2000-292686, 2001-305415 and 2003-153291, a small aperture size of an aperture stop provided in an image taking optical system (the aperture size corresponds to an exit pupil diameter of the image taking optical system) makes an amount of light reaching the focus detection pixels insufficient. In consequence, the image pickup apparatus cannot perform good phase difference detection method focus detection.
In particular, in a case of performing the phase difference detection method focus detection between plural image capturing operations each being performed for capturing one frame image in continuous image capturing, when an aperture size of the aperture stop set for image capturing is smaller than that corresponding to a light amount necessary for the phase difference detection method focus detection, the image pickup apparatus cannot perform good focus detection between the image capturing operations.